Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. Based on the determined class of the PD, the PSE applies the required power to the PD.
In typical 802.3af PoE application, a PSE has active current control in a low-side power supply line between the PSE port and a more negative power supply lead of a pair of power supply leads. This current control provides functions of current limiting and circuit breaking. To meet safety requirements, a fuse or positive temperature coefficient (PTC) device is conventionally used in a high-side power supply line between the PSE port and a more positive power supply lead of the pair of power supply leads, to protect against excessive current in the event of failure of the low-side current limiting.
The problem with this configuration is that over a large temperature range, the fusing current of the fuse or PTC device varies greatly. The fuse or PTC device must guarantee fusing at the current mandated by safety standards when the ambient temperature is very cold. The current that this device can reliably carry without fusing at a high ambient temperature is significantly less.
To support high-power PoE systems, current levels in a PoE should be increased. When current levels are increased, the margin between the fuse current and the safe operating current is reduced to a point at which it is difficult to guarantee maintaining proper fuse operation over a large temperature range.
Therefore, it would be desirable to create an advanced current control mechanism that would reliably support high-power PoE operations over a large temperature range.